For the power supply used in a personal computer, the power supply is configured to provide a variety of output voltages to power various electronic products conformable to miscellaneous input voltage specification. Therefore, the switching power supply used in a personal computer generally can provide output voltages with different levels, for example, 5V or 12V, to power the peripheral devices located within the personal computer.
For a multi-output power supply, more particularly a multi-output DC-DC converter, the load to which the power supply is connected is not always stationary. When the load connected to an output terminal is changed, the output voltage of the other output terminal will be varied in response to the variation of the output current at the load-varying output terminal. In order to fix the deviation of the output voltage as result of the load change, cross-regulation is necessary to balance the output voltages between the multiple output terminals. Therefore, the static regulation performance of the converter will be an important consideration when it is desired to design a converter for providing two or more output voltages.
In a typical multi-output DC-DC converter, it is known that the major inductive elements of the converter, such as transformer and output inductors, and the conduction loss generated in the converter are the major contributors to the cross-regulation. Taking a two-output DC-DC converter as an example, when an output terminal is connected to a light load and the other output terminal is connected to a heavy load, the output current of the output terminal to which the light load is connected is prone easy to enter a discontinuous conduction mode (DCM). Under this condition, the cross-regulation performance of the converter would be deteriorated. Therefore, an efficient solution to improve the cross-regulation performance of a multi-output DC-DC converter is to reduce the output current ripple of the multi-output DC-DC converter, so that the outputs currents flowing in the output terminals can be mutually equal and thus the output current flowing in the output terminal to which the light load is connected is difficult to enter the discontinuous conduction mode.
Based on the foregoing concepts, the conventional cross-regulation technique is carried out by respectively placing an output inductor made up of a coupled inductor on each output channel in order to reduce the output current ripple, so that the output current to which the light load is connected is not prone to enter the discontinuous conduction mode. However, the leakage inductance of the output inductor placed on the output channel is not variable along with the load change. The past experimental statistics and theoretical analysis have proven that the smaller the leakage inductance of the output inductor at the output terminal to which a heavy load connected is and the lager the leakage inductance of the output inductor at the output terminal to which to a light load connected is, the better the cross-regulation performance of the multi-output DC-DC converter can offer. Therefore, if it is desired to impose a strict requirement on the cross-regulation performance of a multi-output DC-DC converter, it would be unsatisfactory to simply rely on the coupled inductor at the output terminal of the multi-output DC-DC converter.
It should be clearly understood from the above statements that if the multi-output DC-DC converter is provided with inductive elements each of which is placed on an output channel of the multi-output DC-DC converter and functioning as a leakage inductance with an inductance being variable according to the change of the output current, the cross-regulation performance of the multi-output DC-DC converter can be significantly improved. The present invention can satisfy these needs.